The present invention relates to a manufacturing method of a sputtering target by subjecting a molten and cast tantalum ingot or billet to processes such as forging, annealing and rolling, and to a tantalum sputtering target obtained thereby.
In recent years, the sputtering method for forming a film of materials such as metal or ceramics has been used in numerous fields such as electronics, corrosion resistant materials and ornaments, catalysts, as well as in the manufacture of cutting/grinding materials and abrasion resistant materials.
Although the sputtering method itself is a well-known method in the foregoing fields, recently, particularly in the electronics field, a tantalum sputtering target suitable for forming films of complex shapes and forming circuits is in demand.
Generally, this tantalum target is manufactured by forging and annealing (heat treatment) an ingot or billet formed by performing electron beam melting and casting to a tantalum material, and thereafter performing rolling and finish processing (mechanical processing, polishing, etc.) thereto.
In this kind of manufacturing procedure, the forging performed to the ingot or billet for the manufacture thereof will destroy the cast structure, disperse or eliminate the pores and segregations, and, by further annealing this, recrystallization will occur, and the precision and strength of the structure can be improved to a certain degree.
For example, the tantalum raw material is subject to electronic beam melting and thereafter cast to prepare an ingot or billet, and subsequently subject to cold forging—recrystallization annealing at 1173K—cold forging—recrystallization annealing at 1173K—cold rolling—recrystallization annealing at 1173K—finish processing to form a target material. In this manufacturing process of a tantalum target, the molten and cast ingot or billet generally has a crystal grain diameter of 50 mm or more.
As a result of subjecting the ingot or billet to hot forging and recrystallization annealing, the cast structure is destroyed, and generally even and fine (100 μm or less) crystal grains can be obtained. Nevertheless, there is a problem in that hetero-phase crystal grains assembled in a form of wrinkles will appear in a part of the target structure after this recrystallization annealing; in particular, a pattern in the form of wrinkles or streaks is formed from the center to the peripheral edge of the target.
When examining the reason such hetero-phase crystal grains assembled in a form of wrinkles are generated in the manufacturing process of conventional technology, even when hot forging and the subsequent recrystallization annealing are performed, primary crystal grains (roughly 50 mm) in the ingot or billet remain, and recrystallized grains will be generated in the primary crystal grains at a recrystallization temperature of merely 1173K (900° C.).
In other words, forging causes the primary crystal grains to be crushed, and, although it appears that they all disappear, at the subsequent recrystallization temperature of roughly 1173K, it is considered that the destruction of the primary crystals is incomplete, and some traces of primary crystals remain. This will not disappear even with the subsequent forging and recrystallization annealing, and it is considered that hetero-phase crystal grains assembled in a form of wrinkles are generated at the stage of the final finish processing.
Since the existence of irregular crystal grains in the target that are generated during forging, rolling or the annealing to be performed thereafter will change the sputtering rate, and there is a problem in that evenness (uniformity) of the film will be affected, generation of arcings and particles will be promoted, and the quality of sputtered deposition may deteriorate thereby. Therefore, it is necessary to suppress the generation of the foregoing hetero-phase as much as possible.
And, when performing sputtering, since it is said that the finer and more uniform the recrystallized structure of the target, and more uniform the crystal orientation thereof, a more uniform deposition is possible, and a film generating few arcings and particles and having stable characteristics can be obtained. Thus, measures for making the recrystallized structure fine and uniform, and arranging it to be in a specific crystal orientation are being taken (e.g., refer to Publication of Translation of International Application No. 2002-518593, U.S. Pat. No. 6,331,233).
When observing the mechanism of recrystallization, generally speaking, a recrystallized structure is an aggregate of individual crystals with respectively different plane orientations, and each crystal is divided by a grain boundary. Before rearrangement occurs, the strain added to the object via plastic working such as cold rolling is absorbed in the primary crystals by the transgranular slip in a certain direction, and the strain is accumulated therein.
Such strained primary crystals take on a network cell structure that is extremely fine with slightly different orientations aggregated with lattice defects such as transition, and are also separated into a plurality of different areas with significantly differing orientations. When this kind of deformation structure is heated, the cells change into subgrains (recovery process) through the combination of transition or rearrangement. The change from a cell into a subgrain hardly involves any change in the measurement. And, it is considered that these subgrains are combined, and a specific subgrain grows to become a recrystallized core, corrodes the non-recrystallized portion, grows and promotes the recrystallization.
As described above, with a tantalum target, it is said that a fully recrystallized structure based on full annealing is favorable in stabilizing the structure.
Nevertheless, with the recrystallization annealing (full annealing) based on a high temperature and to be performed for a long period of time, there is a problem in that the crystal grain size will become coarsened, and the average crystal grain size would ordinarily be 100 μm or more.
When sputtering is performed with a tantalum target having such a coarse recrystallized structure, there are problems in that the evenness (uniformity) of the film will become inferior, the generation of arcings and particles will be promoted, and the quality of sputtering deposition will deteriorate.